1. Field of the Invention
The present invention relates to an image forming apparatus wherein a toner image formed on an image carrier is transferred to a sheet through an intermediate transfer medium.
2. Description of the Related Art
Image forming apparatus capable of printing color images electrophotographically, e.g. color printers, have become widespread.
In this type of color printer, the surface of a photosensitive drum serving as an image carrier is scanned with a laser beam, whereby an electrostatic latent image is formed on the surface of the photosensitive drum. The electrostatic latent image is sequentially developed with a yellow toner (developer), a magenta toner, a cyan toner, and a black toner. Each time development with toner of one color is completed, a developer image, i.e. a toner image, on the photosensitive drum is transferred (primarily transferred) to a transfer belt serving as an intermediate transfer medium. In this way, a full-color toner image is superimposedly transferred to the transfer belt.
When a black and white image is to be printed, an electrostatic latent image formed on the surface of the photosensitive drum is developed with only the black toner. After the development, the black toner image on the photosensitive drum is transferred (primarily transferred) to the transfer belt.
After the full-color toner image or the black toner image has been transferred to the transfer belt, a single sheet serving as an image forming medium is fed to the transfer nip between the transfer belt and a secondary transfer roller. At this time, a bias voltage for transfer is applied to the secondary transfer roller. The application of the bias voltage produces an electric field for transfer extending over from the secondary transfer roller to the transfer belt. The electric field allows the full-color or black toner image on the transfer belt to be secondarily transferred to the sheet.
A sheet that is to be transported to the transfer nip is fed from a sheet cassette by a feed roller through a regist roller serving as a transport roller. In this type of image forming apparatus, the regist roller is generally provided at a position closer to the frame of the apparatus than the transfer nip with a view to making the apparatus compact in size. The secondary transfer roller, which forms the transfer nip, is also provided closer to the frame of the image forming apparatus than the transfer belt.
Meanwhile, the transfer electric field emitted from the secondary transfer roller extends as far as the periphery of the transfer nip. Therefore, some problems have been experienced with the conventional image forming apparatus having the above-described positional relationship between the transfer nip and the regist roller. That is, when fed into the transfer nip, the sheet is transported thereto from a position closer to the secondary transfer roller than the transfer belt. Therefore, a part of toner on the transfer belt may be scattered toward the sheet to adhere to the surface of the sheet before it reaches the transfer nip. The undesired adhesion of toner causes noise and contamination, resulting in degradation of the image quality of the finished print.